ABSTRACT Rapid weight gain during the first two years of life predisposes to later-in-life obesity. However, there is a fundamental gap in knowledge of what dietary recommendations should be provided to infants and toddlers to promote optimal growth and prevent rapid weight gain. Dietary protein intake is considered a risk factor because it accelerates weight and fat gain in infants and toddlers, based primarily on research of dairy protein. However, the underlying mechanisms are not clear. These findings have been the basis of recent recommendations to reduce protein intake from birth to 24 months. What is not known is whether all protein sources have the same weight-accelerating effect. Our research group conducted a randomized controlled trial in infants consuming a meat- or dairy-based complementary diet from 6 to 12 months of age. Growth data showed that a dairy-based complementary diet, not meat, leads to impaired linear growth and increased risk for being overweight. The objective of this proposed project is to investigate the underlying mechanisms and determine which mediators are predictive for the different growth patterns in this cohort via fecal microbiome/metagenomics, serum metabolomics and doubly labeled water technique. Although the candidate has expertise in pediatric nutrition and conducting randomized controlled trials, training in mechanistic investigations of obesity development and research methodologies are critical to achieve the long-term goal of becoming a leader in pediatric nutrition focusing on implementing effective interventions early in life for optimal growth and obesity prevention. This career development application proposes a 4-year multidisciplinary training program to the candidate, who is a 4th year postdoctoral fellow transitioning to a junior faculty position. A well-developed training plan by the candidate and the mentoring team is in place, including 1) Hands-on experience of microbiome/metagenomics, metabolomics analysis and data interpretation; 2) Hands-on experience of conducting the doubly labeled water protocol for body composition and total energy expenditure in infants and toddlers, plus sample analysis and data interpretation; 3) Regular meetings with mentors to monitor project progress, achievements towards the short- and long-term goals, and didactic trainings; 4) Advanced coursework in microbiome, metabolomics and biostatistics; 5) Training in leadership and program management. University of Colorado Anschutz Medical Campus is world-leading research institution and provides the candidate an excellent research and training environment which facilitates a successful transition to an independent career in pediatric nutrition and early-in-life obesity prevention.